Electroacoustic diaphragm coupling



Jan. 30, 1951 2 Sheets-Sheet 1 Filed 001:. 22, 1946 F/Gl 4 Inventor 3mm 1511M 5z Attorney Jan. 30, 1951 D. E. SKELTON ELECTROACOUSTIC DIAPHRAGM COUPLING 2 Sheets-Sheet 2 Filed Oct. 22, 1946 Patented Jan. 30, 1951 ELECTROACOUSTIC DIAPHRAGM COUPLING Dennis Edward Skelton, London, England, assignor to International Standard Electric Corporation, New York, N. Y.

Application October 22, 1946, Serial No. 704,965 In Great Britain August 2'7, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires August 27, 1965 5 Claims.

This invention relates to electro-acoustic devices and particularly to the method of coupling parts of the moving systems of such devices.

One object of the invention is to increase the allowable tolerances in manufacturing such a device.

Another object of the invention is to provide a method of coupling two co-operating parts of the moving system such that the electro-acoustic device will operate with substantially unimpaired efficiency when the relative rest positions of the two parts are changed.

One feature of the present invention comprises an electro-acoustic device in which two co-operating parts of the moving system are so formed that contacting portions of the parts are stiff in the direction of movement, that there is flexibility between the contacting portions normal to the direction of movement, and that the static pressure exerted between the two portions for a range of relative positions varies to such a small extent that the steady deflection, and conse quently the stifiness, of the moving system does not vary substantially over said range;

A second feature of the invention comprises an electro-acoustic device in which two co-operating parts of the moving system are so formed as to be capable of adjusting themselves to a range of relative positions Without substantial distortion of either part from its free rest position and without consequent substantial variation of the stiifness of the moving system.

The two parts of the moving system will usually be a diaphragm and a front electrode but might also, for instance, be two parts of a compound diaphragm.

The invention is accomplished by forming flexible serrations in the edges of a center aperture, or in the outer periphery, of one of the members in a direction substantially parallel to the direction of movement of the two members. The serrated portion of this member is fitted against substantially parallel portions of the other member so that the serrations exert a pressure against the second member in directions Figs. 2-7 show alternative constructions of diaphragms and electrodes.

Referring next to Fig. l, the electrode I has a central domed portion 2, although this might be of any suitable concave form.

The diaphragm 3 is apertured at the centre and formed with a substantially cylindrical central portion 4, the edge of which is serrated to form teeth 5.

The diameter of the cylindrical portion] is slightly smaller than the diameter of the dome 2, so that the cylindrical portion fits within the dome with the teeth engaging its inner surface.

Ideally the contacting surfaces of the two parts;

in this case diaphragm and electrode; would both be parallel to the direction of movement, and so dimensioned that the male member fricti'onally fits within the female member. The extent to which the male member enters the female member during assembly could then vary in accordance with easy manufacturing limits without varying the pressure between the two members.

The use of two such cylindrical surfaces would however of itself introduce fine manufacturing limits and therefore the contacting surfaces are designed so that they are at a small angle to one another but only vary a few degrees from the true cylindrical form.

Due to the flexibility of the teeth 5 normal to the axis deflection can take place to allow a range of positions of part 4 in the dome 2 with little increase of pressure between the parts. Increase of pressure between the parts increases the stiffness of the mechanical system because in such structures stiffness increases rapidly with deflection on account of the tensions introduced in addition to the bending stresses which control the stiffness when the deflection is very small. Increase of stiffness resulting from deflection is undesired because it is difficult to control, and in the present construction this is avoided because due to the flexibility of. the teeth 5 normal to the axis part 4 may have a range of positions in dome 2 without appreciable pressure in the axial direction or appreciable deflection of either part from its free rest position.

, This eases the axial limits necessary in the manufacturing the contacting members. A degree of manufacturing tolerance is also possible in the circular form of the teeth 5 and the dome 2, and in the concentricity of these parts or of the diaphragm and electrode seatings.

The two vibratable members, diaphragm and electrode should be coupled together in such. a

The contacting surfaces should be as near to parallelism with the direction of movement as will permit of ease of manufacture, but, of course,

the absolute and relative slopes of the contacting. surfaces can vary so long as there would .beno

appreciable relative vibration of the parts coupled.

The contact between the surfaces-is such that relative motion takes place when the forces caused by deflection in the direction of driveexceeds the static friction which is determined by the nature of the surfaces, the angle of the surfaces to the direction of drive, the angle'between the surfaces, and the component force between thezsurfacess normal .to' the direction of drive. Such relative motion will take. place when the deflection of :eitherpart from; its. individual rest positionreaches:a valueof. the order of. 1 mils in the embodiment described; and the absolute-deflection that can occur is thereby limited. The desired vibratile deflections *of-the two parts moving as one have an amplitude of a fraction of a mil and for such amplitudes no relative motion or slipping will take'place.

It :will'be seen that i such anarrangement not onlyrallows of; easy manufacturing limits but is alsoicapable of taking'up changes inthe relative static positions of the two parts due to variations inrpressure differences between the air chambers onzopposite sides ofthe diaphragm and electrode respectively; for'instance the'variations likely to beaencountered'when the electrode chamber is effectively: sealed.

While the facility of relative movement of the surfacesafter assembly is useful in certain conditions,: these conditions are not universally required to be met in the design of electro-acoustic devices. When the facility is not required, it may bedesirable to put cement of a liquid or semiliquid kindwhich'subsequently sets hard between the contacting: surfaces during assembly so that when-the parts have taken up their optimum relative positions, the compound will unitethe contacting surfaces'permanently.

In Fig. 1, such an operation will stick the teeth Etc: the interior of the dome 2 from tip to root thus eliminating. an acoustic leak between the frontand back of the diaphragm. during vibration, at acoustic frequencies and preventing passage of breath moisture.

It may'be preferred to useviscous compounds whichwill allow of' slow adjustment of relative positions; such compounds will readily transmit vibrationsat acoustic frequencies. The use of a viscous compound in place'of cement permits satisfactory operation with less pressure between the contacting surfaces. in a direction normal to the direction of drive and thereby allowsv the device to operate witl'na. smaller limiting deflection than the-1v 5 mils. mentioned above.

In Fig. 2, the electrode is in two parts, a flexible base member I having a truncated conical central portion 2 into-which the toothed cylinder 4, fiofrthediaphragm (Hits. The active portion of the .electrode is-formed by a dome 6, preferably of;carbonized nickel.

In Fig. 3 the truncated conical portion 2 of the electrodeof Fig.:2.-is.formed on the front-side .ofrjthe electrode instead 0ft: the back and the members 2, 4 have opposite slopes relative to the axis. The teeth in this case are on the part 2 of the electrode instead of on the diaphragm. As in Fig. 2, the electrode is compound, the active surface being'providedsby a:.-dome-shaped member s preferably of carbonised nickel.

In the arrangement of Fig. 4, the diaphram is similar to that in Figs. 1 and 2, but the electrode is compound, comprising a disc I centrally apertured to receive a thimble 6 having a domed active surface and an extension 2 in front of the disc I to. provide an extended interior surface of constant slope for contact with the teeth 5.

The arrangement of Fig. 5 is similar to that of Fig. 3, the difference being that the teeth are on the diaphragm instead of on the electrode and the teeth areexterior to the reversely-sloped part 2 of the electrode.

In Fig. 6, the invention is applied, not between diaphragm and electrode, but between two parts of arcompounddiaphragm. The 2 outer part 1 I of the diaphragm comprises an. annulus 'l with .a

truncated conical internalextension within which.

bears. asubstantially cylindrical ring-of. teeth 5 on a'.coni'cal. portion 3. The conical portion-is.

centrally apertured and formed with a lip 4 having a smaller angle tothei axisv and. bearing on the base of the dome?! of electrode I.

In Fig. '7, the inventioni is again applied be tween the diaphragm andelectrode. The diaphragm 3' has a central,truncated-conicalexetension 4 without teeth. The compound electrode comprises a disc I and a domed member-'6, from the base flange B of which are cut'a series of. teeth. Alternate teeth:9.are.used in conjunction.

with the flange 8'for fixing the domed member contacting members has-had flexibility normal .to

the direction of movement; It is within the scope of the invention that both contacting members should contribute to such flexibility.

What is claimed is:

1. An electro-acoustic diaphragm comprising an electrode, a diaphragm, said electrode andsaid diaphragm both movable in the same direction, a portion of said electrode overlying a portion of said diaphragm, said electrode and said diaphragm each having a portion extending substantially parallel to the said direction of movement, said diaphragm having a plurality of serrations around an edge of its extended portion, said serrations engaging a portion of the inner surface of the extended portion of said 'electrode permitting maximum friction between said diaphragm and said electrode in a direction parallel to the direction of-movement thereof and maximum resilience between said diaphragm and said electrode in a plane normal to the direction of movement thereof.

2'. An electro-acoustic diaphragm comprising an electrode, a diaphragm, said electrode and said diaphragm both movable in the same direc-' tion, a portion of said diaphragm overlying a portion of said electrode, said diaphragm, and said electrode each having a portion extending substantially parallel to the said direction of movement, said electrode having a plurality of serra tions around an edge of its extended portion, said serrations engaging a portionlof the inner surface of thesaid extended portion of said diaphragm, permitting maximum friction betweenxsaid diaphragm and said electrode in a direction parallel to the direction of movement thereof and maximum resilience between said diaphragm and said electrode in a plane normal to the direction of movement thereof.

3. An electro-acoustic transducer that comprises a substantially conical diaphragm having its apex facing inwardl means for supporting said diaphragm around its outer margin in a manner permitting substantially free axial motion, an otherwise fiat diaphragm having a domelike formation near the center thereof mounted at its margin and arranged relative to the conical diaphragm in a manner such that it can move in a direction common to the motion of the conical diaphragm, said diaphragms being disposed so that apex portions of the frusto-conical diaphragm are within and arranged coaxially relative tothe dome formation of the otherwise fiat diaphragm, the improvements that comprise means for coupling the frusto-conical diaphragm and the diaphragm having the dome formation whereby the diaphragms move in unison and are rigidly coupled for motion along the axis of the frusto-conical diaphragm but are flexibly coupled for motion normal to such axis, said coupling means comprising a plurality of flexible elements formed on marginal portions of the frusto-conical diaphragm near its apex engaging with the inside of the dome-like formation of the a otherwise fiat diaphragm for transmitting motion thereto.

4. An electro-acoustic transducer as defined in claim 3, further characterized in that the flexible elements constituting the coupling means are serrations formed on the edge of the frusto-conica'l diaphragm frictionally engaging with and fitting closely against inner portions of the domelike formation of the otherwise fiat diaphragm.

5. An electro-acoustic transducer as defined in claim 3, further characterized in that the flexible elements constituting the coupling means are serrations formed on the edge of the frustoconical diaphragm, adhesively attached at their end portions to inner portions of the dome-like formation of the otherwise flat diaphragm.

DENNIS EDWARD SKELTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,846,351 Murklam et a1 Feb. 23, 1932 1,848,433 Pridham Mar. 8, 1932 1,869,230 Timmons July 26, 1932 1,930,328 Tichenor Oct. 10, 1933 2,043,833 Bennett June 2, 1936 

